Traditionally, retroreflective microstructures are fabricated using machining methods to create a master tool. Machining retroreflective microstructures often involves the use of a single-point diamond tooling to machine the microstructures onto a suitable blank substrate (see, e.g., U.S. Pat. No. 6,253,442). Flycutting or ruling is typically used to machine the substrate in order to achieve the levels of precision required for retroreflective sheeting specifications (see, e.g., U.S. Pat. No. 5,156,863). During the flycutting process, a high-speed spindle rotates the single-point diamond tool about an axis as the tool traverses along the linear axis of the machine tool. Using this method, cuts are made in the substrate from one side of the substrate to the other at a spacing based on the desired final geometry of the retroreflector. Significant drawbacks to the flycutting method include variations in the final master tool due to wear of the diamond tooling, disturbances in the pattern due to the machine tool, and the length of time required to cut the many grooves required to create the retroreflector.
Recent advances in machining technology include diamond machining retroreflectors on thin shims. These methods, however, are very time intensive, require precise machine tools, and often result in defects to the retroreflectors due to variations of the shims. In particular, variations in the size of shims create variations in height and gaps between the shims when the shims are stacked together for electroforming. These variations and gaps cause staining on the electroformed replicas. Diamond-machining retroreflectors on thin shims also has a disadvantage due to the fragile nature of the shims. Finally, there are limitations to the types of geometries that may be fabricated using this process.
Pin-bundling techniques have also been used to create retroreflective microstructures (see, e.g., U.S. Pat. No. 4,243,618). Pin-bundling techniques are similarly time intensive as the pins must be machined using single-point diamond tools and may result in variations resulting from the tooling and fixturing used. The machining of the pins further limits the geometries of the retroreflective microstructures that may be produced by this method. Staining and electroforming cosmetic issues are also a problem with this technique.
Further, typical methods of fabricating retroreflective microstructure tooling involve creating a master by electroforming and replicating that master many times into opposite generation electroforms. The multiple electroforms are tiled or parqueted together to create a finished tool of a large enough size to enable the production of retroreflective film. This process is time intensive and may take many months to years to complete depending on the complexity of the design and the size of the finished tool needed to make the sheeting.
As such, there is a need for a method of fabricating retroreflective microstructures in a more time efficient manner. There is also a need for a method of fabricating retroreflective microstructures that contain geometries that allow for enhanced optical features. Further, there is a need to provide retroreflector tooling that can create a continuous retroreflective sheeting that eliminates the need to tile or parquet multiple electroforms to create a finished tool of a large enough size to enable production of retroreflective film. This technology is directed to overcoming these and other deficiencies in the prior art.